1. Field of the Invention
The present invention generally relates to, and more particularly to image processing apparatuses and computer-readable storage media, and more particularly to an image processing apparatus that codes image data hierarchically, and to a computer-readable storage medium that stores a program for causing a computer to carry out such a hierarchical coding.
2. Description of the Related Art
Due to improvements made in image input techniques and image output techniques, there are increased demands to improve the high definition property of color still images. One example of an image input apparatus is a digital camera (DC), and helped by reduced cost of high-performance charge coupled devices (CCDs) having 3,000,000 or more pixels, such high-performance CCDs are widely used for digital cameras in the popular price range. It is expected that products using CCDs having 5,000,000 or more pixels will be put on the market in the near future. This trend of increasing the number of pixels of the image input device is expected to continue.
On the other hand, considerable improvements have also been made in image output devices and image display devices, such as hard-copy devices including laser printers, ink jet printers and thermal printers, and soft-copy devices including flat panel displays made of cathode ray tubes (CRTs), liquid crystal displays (LCDs) and plasma display panels (PDPs). The high definition property of such image output devices and image display devices have improved considerably, and the cost has greatly been reduced.
Because of these high-performance and inexpensive image input devices and image output devices on the market, the use of high-definition images have become popular. It is expected that the demands for high-definition images will increase in the future in various fields. Actually, the developments in personal computers (PCs) and network-related technologies including the Internet have accelerated such trends. Especially in recent years, mobile equipments such as portable telephones and laptop computers have become extremely popular, and there are more and more opportunities to transmit or receive high-definition images via a communication unit.
Consequently, it is expected that the demands to further improve the performance or function of the image compression and/or expansion techniques will increase in order to facilitate processing of the high-definition images.
As one of image compression and expansion algorithms for facilitating the processing of such high-definition still images, the JPEG (Joint Photographic Experts Group) system is popularly used. In addition, the JPEG2000, which has become an international standard in 2001, uses an image compression and expansion algorithm with a high performance that is further improved compared to the JPEG. The JPEG2000 divides the image into rectangular regions called tiles, and is capable of carrying out the compression and expansion process under a relatively small memory capacity environment. In other words, each tile becomes a basic unit of the compression and expansion process, and the compression and expansion process can be carried out independently for each tile. In addition, according to the JPEG2000, it is possible to separate a low-resolution data and a high-resolution data within one image file.
However, the algorithm of the JPEG2000 is complex, and the load on the coding process and the decoding process is large compared to that of the conventional JPEG. For example, the load on a server computer is large even in a case where the image files are compressed according to the JPEG2000 and stored in the server computer, a client computer makes access to the server computer to extract only the low-resolution data from the compressed image files when necessary to make a thumbnail display process.